When we allocate a graphene_point_t on the stack, there's no guarantee
that it will be aligned at an 8-byte boundary, which is an assumption
made by gsk_pathop_encode() (which wants to use the lowest 3 bits to
encode the operation). In the places where it matters, force the
points on the stack and embedded in structs to be nicely aligned.
By using a distinct type for this (a union with a suitable size and
alignment), we ensure that the compiler will warn or error whenever we
can't prove that a particular point is, in fact, suitably aligned.
We can go from a `GskAlignedPoint *` to a `graphene_point_t *`
(which is always valid, because the `GskAlignedPoint` is aligned)
via &aligned_points[0].pt, but we cannot go back the other way
(which is not always valid, because the `graphene_point_t` is not
necessarily aligned nicely) without a cast.
In practice, it seems that a graphene_point_t on x86_64 *is* usually
placed at an 8-byte boundary, but this is not the case on 32-bit
architectures or on s390x.
In many cases we can avoid needing an explicit reference to the more
complicated type by making use of a transparent union. There's already
at least one transparent union in GSK's public API, so it's presumably
portable enough to match GTK's requirements.
Increasing the alignment of GskAlignedPoint also requires adjusting how
a GskStandardContour is allocated and initialized. This data structure
allocates extra memory to hold an array of GskAlignedPoint outside the
bounds of the struct itself, and that array now needs to be aligned
suitably. Previously the array started with at next byte after the
flexible array of gskpathop, but the alignment of a gskpathop is only
4 bytes on 32-bit architectures, so depending on the number of gskpathop
in the trailing flexible array, that pointer might be an unsuitable
location to allocate a GskAlignedPoint.
Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/6395
Signed-off-by: Simon McVittie <smcv@debian.org>
There is no decomposition going on for any contours,
and the tolerance argument is entirely unused.
Decomposition and tolerance is handled entirely
in gskpath.c by its trampoline.
Add a contour that optimizes some things for
rectangles. Also add rectangle detection to the
path parser, and add tests similar to what we
have for the other special contours.
This special contour takes advantage of its
rounded-rect-ness for speeding up bounding
boxes and winding numbers. It falls back
to the standard contour code for everything
else.
In order to compute path lengths efficiently, we need
to cache lookup tables. This commit adds API to let
contours allocate and free such measure data, as well
as API to use the data to go length -> point and
vice versa.
This commit adds the basic infrastructure for paths.
The public APIs consists of GskPath, GskPathPoint and
GskPathBuilder.
GskPath is a data structure for paths that consists
of contours, which in turn might contain Bézier curves.
The Bezier data structure is inspired by Skia, with separate
arrays for points and operations. One advantage of this
arrangement is that start and end points are shared
between adjacent curves.
A GskPathPoint represents a point on a path, which can
be queried for various properties.
GskPathBuilder is an auxiliary builder object for paths.
